The present invention relates to a transducer or converter for converting an electric input signal into an electric signal for mechanical output and more particularly relates to a transducer or converter which utilizes a winding and a circuit connected thereto to produce a negative source impedance.
There are various typical constructions known for such transducers, depending upon the type of output desired. If the output signal is an electric signal, then the transducer typically is a transformer. The input signal, which has a specific current/voltage ratio is converted by the transformer into an output signal with a different current/voltage ratio. If the output takes mechanical form such as movement or force, then the transducer can be an electrodynamic transducer or loudspeaker, or an electric motor. In any of these cases a winding is disposed in a magnetic field and a current flowing through the winding, produced for example by the input signal, causes a movement of the winding in the magnetic field. This movement can produce as an output an oscillation or a continuous movement. In many transducer applications, it is desired or even required for the output changes to follow the input signals as closely as possible so that such transducers can have a high degree of linearity or proportionality between the input signal and the output.
For example, in the case of a loudspeaker, it is desirable for the mechanical oscillations of the diaphragm to closely, if not exactly, correspond as proportional to the oscillations of the electric input signal. If this is not the case, the loudspeaker has poor fidelity. Poor fidelity can result from the fact that the current carrying winding in the magnetic field in which it is located is subject to mechanical restoring forces which destroy or modify the actual movement thereof.
When the transducer is a transformer, it is desirable for the electric signal in the secondary winding to be as precisely proportional as possible to the signal in the primary winding. However, this is only the case if the transformer is operated in a range for which its transfer characteristic is linear. The transfer characteristic is not linear when an iron core is used, since the magnetizing current required by the iron core is not linear.
When the transducer is a direct current motor, it is known that the linearity relationship between the output (rotary signal) and input signal will be impaired by the mechanical loading.
European patent application No. 0 041 472 discloses a circuit arrangement, particularly with an electromagnetic or electromechanical transducer which employs an iron core. The signal is modified in a nonlinear manner because of the nonlinear characteristic of the core but the circuit produces signal compensation whereby the linearity is restored. For this purpose, the voltage drop in the winding resistance of the inductor is measured and is compensated for by applying an additional voltage which eliminates the effect of the voltage drop.
This additional voltage is produced across a resistor, which is connected in series with the winding (which has resistive as well as inductive parameters). However, in order to be able to fulfil its function in an optimum manner, the resistor should be made from the same material and have the same operating temperature as the winding. As a result, the resistor must be positioned as close as possible to the winding, which is often not possible. In a moving-coil loudspeaker, for example such a resistor cannot be positioned close enough to the moving-coil, because there is no space available. Consequently there are many applications in which a compensating resistor cannot be used whereby the undesired nonlinearities of the transducer cannot be eliminated.